Generally speaking, an electric machine comprises a stationary part, commonly referred to as “stator” (or “stator assembly”), and a mobile part, both equipped with windings of electrical conductor and/or sources of a magnetic and/or electromagnetic field. Together with the machine structure, these windings and sources always form both an electric circuit (defined as an assembly of structures and materials with an electric current and/or an electric field flowing through it) and a magnetic circuit (defined as an assembly of structures and materials with a magnetic field flowing through it). In order to operate, the electric machine uses electromagnetic induction (produced by the linkage of magnetic field fluxes with the electric windings) and/or electromagnetic forces (generated by the magnetic/electromagnetic field sources on the electric windings with current flowing through them and/or by the other magnetic/electromagnetic field sources). Some electric machines (for example, electric motors) can convert the electric current circulating in the electric windings into a movement of the mobile part relative to the stator. Other electric machines (for example, generators) can generate electric current and/or electrically driving forces in the electric windings using the motion of the mobile part relative to the stator. An electric machine of this kind can normally be used in both ways (that is, as a generator and as a motor). The windings can be made around a core of magnetic material in order to optimize the effect of the linkage of the magnetic flux with the electric windings themselves .
In one type of electric machine, the mobile part is a rotary member, also known as “rotor” (or “rotor assembly”). The axis of rotation of the rotor is particularly important and is usually used as the reference and/or symmetry axis for the structure of the electric machine. As the rotor moves relative to the stator, portions of the magnetic field sources and portions of the electric windings face each other at a given distance defining a gap between the rotor and the stator. There is a geometrical relationship between the axis of rotation of the rotor and the way in which the streamlines of the magnetic field, generated by the sources, are arranged in the gap between the stator and the rotor. Based on this geometrical relationship, machines of this kind can be divided into two categories: radial-flux electric machines and axial-flux electric machines. In a radial-flux electric machine, the arrangement of the magnetic field sources and of the electric windings, with which the magnetic field is linked, is such that, in the aforesaid gap between rotor and stator, the streamlines of the magnetic field can be approximated with segments stemming from straight lines that are perpendicular to the rotation axis of the rotor and are arranged in a radial manner relative to the rotation axis itself. In an axial-flux electric machine, the arrangement of the magnetic field sources and of the electric windings, with which the magnetic field is linked, is such that, in the aforesaid gap between rotor and stator, the streamlines of the magnetic field can be approximated with segments stemming from straight lines that are parallel to the rotation axis of the rotor.
Knowingly, axial-flux electric machines have a specific architecture, in which the windings and/or the stator and rotor permanent magnets are arranged on respective parallel discs, which are arranged close to one another and are separated by a gap, whose thickness extends in the direction of the rotation axis of the rotor. Therefore, this architecture is remarkably different from the one of radial-flux motors.
For this type of motors, two main architectures are known, the first one comprising a single rotor interposed between two fixed stator discs, the other one comprising a fixed stator disc interposed between to rotor discs. Other architectures are also possible, which, in particular, are obtained by combining - in a modular manner - a plurality of units of one specific architecture or the other, as described above. Generally, the rotor of axial-flux electric machines has permanent magnets, whereas the stator comprises a ferromagnetic core with a toroidal shape, on which coils are fitted, which link the rotor magnetic field.
In other cases, which are particularly interesting for the present invention, it is the stator that has permanent magnets, whereas the rotor comprises a ferromagnetic core with a toroidal shape, on which coils are fitted, which link the rotor magnetic field. In this case, the rotor is defined as a “winding rotor”.
With reference to the case in which the rotor is a winding rotor, more in detail, the coils are alternated with the same number of teeth made of ferromagnetic material, which define the aforesaid gap relative to the stator and which, in cooperation with the ferromagnetic core, determine the rotor section of the streamlines. In other words, the teeth extend from the toroidal core parallel to the rotation axis of the rotor itself and, between them, the slots are defined, which house the coils.
A known solution to manufacture a winding rotor comprises, first of all, manufacturing the core of the rotor by joining a succession of punched metal sheets, which define the final outline of the body made of ferromagnetic material, which already has the teeth and the slots in between them. Subsequently, the coils are manufactured by hand-winding the copper conductor in the respective slots, thus completing the production of the winding rotor.
If, on the one hand, this system can grant to the rotor a remarkable mechanical resistance, we would like to point out that, on the other hand, the manufacturing process becomes more complicated, due to the manual operation needed to wind the coils.
This negatively and significantly affects the costs for the production of the electric machine.
Another solution to manufacture a toroidal core provided with windings comprises cutting in half a smooth ferromagnetic core, with a toroidal shape, in order to obtain to half-rings, so as to subsequently fit on each half-ring the coils that were manufactured before. However, this solution, which can be carried out in a simple and low-cost fashion (the coils can be obtained through simplified industrial procedures), does not use teeth made of ferromagnetic material. This leads to a first drawback concerning a high dispersion of the flux, which is not conveyed correctly, thus causing the machines to deliver insufficient performances.